Unadorned timber poles have a very limited compression capability in the direction of the timber grain.
In mines in which the hangings close with the footwalls, various expedients have been resorted to prolong the load supporting life of plain wooden props. The most useful methods of stretching the life of a wooden pole is by the use of a cross-grain head boards and tapered ends which only slightly increase the degree by which the props may be decreased in length under compression. A fairly recent innovation which considerably increases the degree of longitudinal compression of a prop is to encase the prop in a sleeve of material such as ductile mild steel which surrounds the prop over a substantial portion of its length and is adapted progressively to release hoop stress built up in the timber element as it is compressed.
A problem with the sleeved props is that although they are quicker to accept load than for example a mat pack their performance characteristics display, after they initially become load supporting, a rapid loss of support before again becoming fully load supporting. There is a possibility that this problem may induce bed separation and open rock joints to destabilize the hanging.